key: cord-0270451-1cdn8fkr
authors: Ismail, Nishana; Ferreira, Placid M.
title: Active Elastomeric Composite Dense Array Stamp For Micro-transfer Printing
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.021
sha: 05924ad7bb11876b46b53fbe5492ce4d08eb7d00
doc_id: 270451
cord_uid: 1cdn8fkr

Abstract Micro-transfer printing is a powerful methodology for the manufacturing of 3D heterogeneously integrated micro-systems. This paper explores an active elastomeric composite dense array stamp which improves throughput while enabling local monitoring and control of the process. This dense array stamp is a scalable 4x4 geometry design with a multiplexed interconnection scheme, ensuring a small footprint. It consists of 16 individual stamps, each with a Lead Zirconate Titanate (PZT) actuator and a strain gauge sensor. Actuation and sensing characterization and closed-loop feedback control were established, and the dense array stamp performance was validated using selective pickup and place micro-transfer printing experiments.

Assembling of individual micro components to yield hybrid microsystems is a special area of interest in microfabrication. The monolithic layer-by-layer manufacturing nature of conventional microfabrication techniques makes 3D heterogeneous device fabrication very difficult, particularly in case of large-scale assemblies involving generally incompatible materials. This becomes a bottle neck for new emerging areas of microfabrication such as flexible electronics, 3D integrated circuit systems and MEMS (Micro-Electro Mechanical Systems) where complex heterogeneous 3D architectures are desired for optimal performance [1] , [2] , [3] . The micro assembly process of transfer printing is a good solution to this problem.

Micro-transfer printing is a fabrication process that uses a viscoelastic stamp to pick and transfer normal microfabricated structures (inks) from handle substrates to a wide variety of receiver substrates (like transparent and flexible polymers). This simple process has the ability to manipulate the fabrication to have the desired heterogeneous integration as well as aerial density [5] . Micro-transfer printing's ability to offer high performance interconnection, high density multifunctional Integration of 3D components makes it a very intriguing approach in for MEMS fabrication. The high throughput rates and inherent ability for parallelism allows this fabrication process to have large scale production in an inexpensive way which is one of the big challenges of microfabrication [6] . Fig. 1 shows several examples of

Assembling of individual micro components to yield hybrid microsystems is a special area of interest in microfabrication. The monolithic layer-by-layer manufacturing nature of conventional microfabrication techniques makes 3D heterogeneous device fabrication very difficult, particularly in case of large-scale assemblies involving generally incompatible materials. [1] , [2] , [3] . The micro assembly process of transfer printing is a good solution to this problem.

Micro-transfer printing is a fabrication process that uses a viscoelastic stamp to pick and transfer normal microfabricated structures (inks) from handle substrates to a wide variety of receiver substrates (like transparent and flexible polymers). This simple process has the ability to manipulate the fabrication to have the desired heterogeneous integration as well as aerial density [5] . Micro-transfer printing's ability to offer high performance interconnection, high density multifunctional Integration of 3D components makes it a very intriguing approach in for MEMS fabrication. The high throughput rates and inherent ability for parallelism allows this fabrication process to have large scale production in an inexpensive way which is one of the big challenges of microfabrication [6] . Fig. 1 shows several examples of

Assembling of individual micro components to yield hybrid microsystems is a special area of interest in microfabrication. The monolithic layer-by-layer manufacturing nature of conventional microfabrication techniques makes 3D heterogeneous device fabrication very difficult, particularly in case of large-scale assemblies involving generally incompatible materials. [2] , [3] . The micro assembly process of transfer printing is a good solution to this problem.

Micro-transfer printing is a fabrication process that uses a viscoelastic stamp to pick and transfer normal microfabricated structures (inks) from handle substrates to a wide variety of receiver substrates (like transparent and flexible polymers). This simple process has the ability to manipulate the fabrication to have the desired heterogeneous integration as well as aerial density [5] . Micro-transfer printing's ability to offer high performance interconnection, high density multifunctional Integration of 3D components makes it a very intriguing approach in for MEMS fabrication. The high throughput rates and inherent ability for parallelism allows this fabrication process to have large scale production in an inexpensive way which is one of the big challenges of microfabrication [6] . Fig. 1 shows several examples of

Assembling of individual micro components to yield hybrid microsystems is a special area of interest in microfabrication. The monolithic layer-by-layer manufacturing nature of conventional microfabrication techniques makes 3D heterogeneous device fabrication very difficult, particularly in case of large-scale assemblies involving generally incompatible materials. [2] , [3] . The micro assembly process of transfer printing is a good solution to this problem.

Micro-transfer printing is a fabrication process that uses a viscoelastic stamp to pick and transfer normal microfabricated structures (inks) from handle substrates to a wide variety of receiver substrates (like transparent and flexible polymers). This simple process has the ability to manipulate the fabrication to have the desired heterogeneous integration as well as aerial density [5] . Micro-transfer printing's ability to offer high performance interconnection, high density multifunctional Integration of 3D components makes it a very intriguing approach in for MEMS fabrication. The high throughput rates and inherent ability for parallelism allows this fabrication process to have large scale production in an inexpensive way which is one of the big challenges of microfabrication [6] . Fig. 1 shows several examples of 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to systems fabricated using transfer printing: (a) Silicon micromasonry [7] , (b) OLED display with printed electronics [8] , (c) Ultrathin inorganic light-emitting diodes [9] , (d) 3D optical negative index metamaterial [10] (e), Ultrathin silicon solar cells [11] , and (f) Epidermal electronics.

The stamps for the micro-transfer printing play an important role in ensuring this high throughput rate. This has led to further research in developing better stamps which can be manipulated during the pickup and placement processes to improve sensitivity and efficiency. These methods include: patterning elastomeric micro-tip adhesive surface in a stamp, enabling switchable adhesion; making bubble stamps, which have programmable control of the interface adhesion through pneumatic pressurization; manipulating dynamic rigidity of shape memory polymer (SMP) to change adhesion controlled stamp adhesion modulation; and laser transfer printing, where laser heating is used for generating a thermal mismatch strain at the stamp-ink interface [12] , [13] , [14] , [15] in a SMP stamp; targeted shear loading for [16] . Some of these processes are illustrated in Fig. 2: (1) Angled stamp [17] , (2) SMP stamp [14] , (3) Bubble stamp [13] , (4) Micro-tip stamp [12] , and (5) Laser transfer printing [16] .

Another strategy to improve the micro-transfer printing is by replacing the passive, bulk Poly(dimethylsiloxane) (PDMS) stamps with active stamps, improving monitoring and control of the interaction between the stamp and the substrates [18] , [6] . This locally active composite stamp gives better control and selectivity in the pick and place processes in the transfer print cycle, allowing new process features such as the ability to use multiple donor and receiver substrates, selective pick-up and printing of inks, high aerial density printing, and increased parallelism and robustness for large scale prints. The active stamp is made of a 3D heterogeneously integrated, functional, multi-layered composite, which consists of usually incompatible materials. This is achieved through a novel fabrication process which is a combination of both conventional MEMS fabrication technologies and micro-transfer printing. The linear array active stamps were built with only 4 devices in a row and the circuitry was established for only one stamp [19] . Hence, only one ink could be printed at a time, with maximum extension possibility of four, considerably limiting the throughput required, especially in large area printing applications. This problem is addressed by designing and manufacturing a scalable, locally active, array stamp that can be customized for required aerial density. The demonstrated solution is an active array stamp of 4x4 geometry with 

The active stamp adds dynamic geometry modulation to the stamp architecture. This is achieved by designing a multilayered elastomeric composite with the needed materials so as to achieve the required functionalities and compliance [5] .

The active stamp architecture has 4 layers [6] as shown in Fig. 3 . These layers are made of the following elements highlighted in the left image of [5] .

The bottom contact post layer is made of PDMS so as to have the same characteristics like adhesion, compliance, mechanical isolation and moldability of a bulk PDMS stamp, generally used in micro-transfer printing [4] . Next is the active layer, which is made of SU8 and patterned as cantilevers. This cantilever pattern helps obtain localized compliance at the posts and to maintain the compliance of the active stamp to be similar to that of the bulk stamp [6] . To obtain the deflection of each cantilever, an actuation element is embedded in this SU8 layer. These elements could be strain gauges, Lead Zirconate Titanate (PZT) etc. Here, the chosen actuation element is a PZT active element with a top gold electrode and a bottom platinum electrode. This choice is based on the ability of PZT to act both as actuator and sensor, high resistivity to environmental factors, low power, fast response, good dynamic characteristics and high load capacity [20] . The PZT is embedded in the carrier layer by using transfer printing on the PDMS layer while it's in a semi cured state and then, patterning the SU8 over it. This use of microtransfer printing for the manufacturing helps selective and accurate placement of the PZT and opens the doorway to explore more complex layer structures.

The active layer is followed by a metal interconnect layer which connects the actuation element with the electric circuitry. This layer is made using patterned copper through photolithography and etching. The embedded sensor layer for feedback control is a strain gauge layer and is made by photolithographically patterning gold. Finally, a handle layer of SU8 is patterned with windows to allow free deflection of the cantilevers, provide access to the copper contact pads as well as give rigidity for easier manipulation.

The active stamp thus formed can be used for better monitoring and control of the micro-transfer printing process as demonstrated by Ahmed et al [5] . In this paper, the above architecture of the composite stamp is extended towards a multiplexed dense array stamp.

By expanding the current architecture to a multiplexed dense array system, this work greatly improves on the scalability of the printing process by increasing the aerial coverage and maintaining the stamp geometry modulation control of each stamp in the array. The dense array of 4x4 geometry was designed to be of a square inch in area, including the interconnects. In order to support a large-scale array fabrication, every step of the fabrication procedure needs to be at optimum performance for high yield and efficiency. Through an analysis of the fabrication methodology, a bottle neck was identified at the PZT ink release step. The PZT inks were fabricated on a Silicon wafer with a thin layer of silicon dioxide in between with a gold top electrode and a platinum bottom electrode. The conventional practice for preparing the PZT inks for micro-transfer printing is by undercut etching the silicon dioxide using Hydrofluoric acid (HF), leaving a small anchor of silicon dioxide holding the ink in place [5] . However, due to PZT's susceptibility to HF [21] , the long undercut etch results in PZT being attacked by the acid, creating pinholes. These pinholes in a PZT ink, as shown in Fig. 4 , results in many of the inks to be short-circuited and hence, the yield of this step only about 50%, making it detrimental for a large array fabrication. Therefore, the HF undercut etch was replaced with a backside dry etch process to preserve the PZT inks. This kind of Silicon-on-silicon dioxide dry etching is carried out by Reactive Ion Etching (RIE) using freon plasma [22] . This dry backside etching removes the handle and undercut layers, leaving behind free standing PZT inks with very few damaged ones. The yield was thus increased to about ~80% to meet the requirement of dense array fabrication.

A sensing element is desirable for these instrumented stamps for feedback and control of the actuation of the active stamp array. The complexity with adding the sensor layer to the array comes from the number of interconnect lines needed for each unit. Each stamp's required interconnect lines go from 2 to 4 on adding a sensor, which makes a total of 64 interconnect lines for the 4x4 array. For improving the scalability of the array design, the concept of multiplexing was used to re-design the interconnect lines [23] . By this methodology, the interconnect lines were multiplexed horizontally and vertically to convert 16 lines to 4, by replacing one line for every 4 lines. For example, in one layer of interconnects, the top electrode input to the PZT of 4 devices in a row were connected to one interconnect line. Then, for the next interconnect layer, isolated from the previous with a dielectric, the bottom electrode interconnect lines of 4 devices in a column were connected. Thus, by actuating row line 2 and column line 4, one can actuate stamp at assigned location (2, 4) . Fig. 5 shows the schematic for the active elastomeric composite dense array stamp with actuators (PZT), sensors (strain gauges), and the multiplexed interconnection scheme. Fig. 6 shows the exploded view detailing the different layers in this architecture. The array thus fabricated includes both actuator and sensor pair on each stamp without increasing the exponential aerial coverage required by the regular increase in interconnect lines. Fig. 7 shows an active elastomeric dense array stamp of 4x4 geometry (left) as fabricated on handle wafer and (right) released and packaged. This interconnection scheme ensures modularity and scalability for the array stamp design.

The dense array stamp is connected to drive circuits and a power source that provide the high voltage for moving the individual stamps. In order to address the stamps individually in the multiplexing scheme, four multiplexer/demultiplexer (MUX/DEMUX) (74HC4051, Sparkfun) were used; two for the actuator inputs and two for the strain gauge output. Each of these boards have three selector pins which can be used to address one of the eight channels by setting them to high or low values. For this device, four channels of one board correspond to the rows of the dense array stamp and four channels of another board correspond to the columns. In this way, by applying a positive voltage to Channel 0 of one board and a ground to Channel 0 of the other board, the cantilever in the (1,1) position is actuated. The convention used here is to apply positive voltage on the row channels and ground on the columns for the PZT actuators, giving a demultiplexing scheme. A similar setup is used to measure output from the strain gauges, where only four channels on each of the two boards to used, in a multiplexing scheme. The multiplexed strain gauge sensor outputs are connected to a Wheatstone bridge and an instrumentation amplifier (AD622, Analog Devices) is used to amplify the output. The amplified signal is then sent into a data acquisition system with an integrated analog-to-digital converter (USB-6009, 14-bit ADC, National Instruments). This signal is then transmitted into the transfer printing tool software [24] The actuation of the stamps were characterized by measuring cantilever deflection as a function of voltage applied. Fig. 8 shows measured deflection vs applied voltage of four devices on the diagonal of the 4x4 array. As shown in the plot, the actuation trend of all the devices are very close to each other. Over 20μm of deflection, the variance between the four cantilevers is a maximum of 3.5%. The average measured actuation was about 0.95μm/V.

The average sensitivity of the strain gauge was measured by plotting output voltage of the strain gauge with respect to the cantilever deflection. Fig. 9 shows the plot of voltage output from the strain gauge with respect to the cantilever deflection for the four devices on the diagonal of the 4x4 array. From this data, the sensitivity of the strain gauge sensor was found to be about 10mV/μm [24] .

A closed loop feedback system would help with better position and force control of the actuation. For this, the actuation and sensor transform functions of the system are calculated by using a Linear-Time Invariant model [25] .

A feedback control law was derived for a single cantilever by Ahmed et al [19] . This control scheme was implemented by first identifying the system transfer function using step response data and the MATLAB® system identification toolbox. This resulted in a second order transfer function. A proportional plus integral (PI) control law was selected for this system. With the identified transfer function, the proportional gain and the integral gain were calculated as 1 Fig. 7 . Active elastomeric dense array stamp of 4x4 geometry. Note: devices (2, 1) and (4, 4) were removed because they suffered damage due to a short circuit, after the experiment. and 72, using PID controller tuning tool in MATLAB®. The schematic for the closed loop control for the system is illustrated in Fig. 10 .

This controller was tested against open loop response for the dense array stamp and Fig. 11 indicates the reference tracking performance measured to validate the closed loop control in comparison to open loop control. Based on this response, the PI control law developed by Ahmed et al. is found satisfactory for the dense array stamp.

To demonstrate the micro-transfer printing process with the dense array stamp, it was used to perform selective pickup and printing of inks. A silicon wafer chip; with inks of size 400 μm x 400 μm area and 3 μm thick, fabricated using standard microfabrication procedure [7] ; was used as a donor substrate. The receiver substrate was a semi-cured PDMS membrane. Fig. 12 shows an example of controlled printing using the dense array stamp. The process used to print each letter of "UIUC" starts with bringing the dense array stamp within 10 μm of the donor substrate. From this position, each cantilever corresponding to the shape of the letter is actuated serially using the DEMUX, described in Section 4. For each cantilever, the control described in Section 5 is applied. Therefore, to pickup the letter "U", the (1,1) cantilever, followed by (2,1) cantilever, then (3,1) cantilever and so on are actuated sequentially, until the entire letter pattern of inks is picked up. Then the entire dense stamp is raised and then translated over to the receiver substrate. Then, the stamp is lowered to conformal contact with the receiver substrate and pulled away, leading to printing of the inks. The speed needed for the print step is considerably slower than ink pickup because fast speeds could lead to the ink remaining on the stamp. Thus, the entire pickup step is comparable to printing step in time length, despite the serial actuation of the cantilevers, since the cantilever actuation is approximately ten times faster than the print step. This process is repeated for each letter, printing "UIUC", as shown.

This work developed the design, fabrication and characterization of an active elastomeric composite dense array micro-transfer printing stamp with both actuation and sensing. The fabrication procedure was optimized for maximum yield efficiency and multiplexing was introduced in the interconnection scheme for modularity and scalability. The dense array stamp was characterized for actuation and sensing, and the system was accordingly calibrated. Closedloop control was established, and the corresponding reference tracking performances were recorded. The active elastomeric composite dense array stamp was also used in micro-transfer printing experiments to demonstrate selective pickup and placement during printing. This enables selective large-scale printing over an area in one step. For future work, the proven architecture can be expanded to a larger array geometry and used in a tactile actuator-sensor application like haptic feedback as an electronic skin membrane. Fig. 13 shows the process flow for the 4x4 dense array with the actuator only. Here, the seven illustrated steps are: (1) SU8 100 (epoxy based negative resist) mold pattern on handle wafer, (2) spin coat PDMS on mold wafer, (3) transfer print PZT from the donor wafer on to the PDMS using PDMS micro stamp, (4) spin coat SU8 on the substrate and pattern cantilevers of SU8, (5) sputter and pattern of Copper for the interconnect layer (6), thick SU8 layer on top as a handle layer (7), device peeled off of mold.

For the multiplexed dense array stamp with actuator and sensor, after Step (5), steps (4) and (5) are repeated to give the two layers of copper interconnects, separated by a dielectric, for actuating the PZT inks. This is followed by sputtering and patterning of Gold for the strain gauge layer. Then, Steps (4) and (5) are done twice to give the multiplexed interconnect (6) and (7) conclude the multiplexed dense array stamp fabrication process flow.

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